AFAIK, the booking engine will not show connections where BD has no fares between the pair of cities, as would be the case between EDI and MME. If you really want to book on flybmi.com, you would have to do it on separate tickets, MME-LHR and then LHR-EDI. Alternatively, go to a TA or use an online booking engine with multi-city capacity (expedia, travelocity, etc...)